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Plexiglass Dead Front

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dbhennessey

Member
Location
New Mexico
Occupation
Electrician
I work for a defense contractor that builds assets for various military contracts. We have to cram a lot into small spaces, so I go around with the engineers on issues such as location, size of enclosures, and other issues. We have our own fabrication department and have the capability to build things in house. An engineer purchased a 14"x6"x6" Nema 12 enclosure to house a Square D 200A 3P circui8t breaker to interrupt the load side of a 75KVA transformer. We fabricated a back plate to mount the breaker on and I suggested a Metal dead front to install in front of this breaker allowing only access to the breaker handle. The engineer decided he wanted a plexiglass dead front instead. My argument that although it makes it finger safe when you open the hinged cover to energize the breaker, It is not a good idea for a possible arc flash. Is there anything in code or anywhere else regarding this practice? I have said I will not sign off on this install.
 
H

Hv&Lv

Senior Member
Location
-
Occupation
Engineer/Technician
dbhennessey said:
I work for a defense contractor that builds assets for various military contracts. We have to cram a lot into small spaces, so I go around with the engineers on issues such as location, size of enclosures, and other issues. We have our own fabrication department and have the capability to build things in house. An engineer purchased a 14"x6"x6" Nema 12 enclosure to house a Square D 200A 3P circui8t breaker to interrupt the load side of a 75KVA transformer. We fabricated a back plate to mount the breaker on and I suggested a Metal dead front to install in front of this breaker allowing only access to the breaker handle. The engineer decided he wanted a plexiglass dead front instead. My argument that although it makes it finger safe when you open the hinged cover to energize the breaker, It is not a good idea for a possible arc flash. Is there anything in code or anywhere else regarding this practice? I have said I will not sign off on this install.
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His argument may be you can IR through some "plexiglass". Or IR acrylic..

You can't IR through metal plate.
 
petersonra

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
dbhennessey said:
The Nema 12 enclosure certainly is. what we work on is not covered by the NEC but we try to accommodate it as much as we can.
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NEMA type enclosures are not listed. UL type enclosures are listed. NEMA 12 looks a lot like UL type 12 but they are not listed to the NEMA 12 standard.

In any case if you replace the four you might as well get what you want out of it

I am surprised you don't build such things to UL508a standards and get them listed.
 
B

brycenesbitt

Senior Member
Location
United States
dbhennessey said:
The Nema 12 enclosure certainly is. what we work on is not covered by the NEC but we try to accommodate it as much as we can.
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Well then.
You get to make it work, rather than just meet a code.

The clear cover would provide excellent protection.
And allow visual inspection. If a wire gets brown, or water gets in or... or....
And talk about arc flash reduction: the clear case lets you see what trouble you're getting into before removing the cover.
...
But I suggest you ask over here:
 
G

garbo

Senior Member
I would not want to be in front of a plexiglass cover in a Arc fault no matter how much PPE I was wearing. We had special IR maybe 3 to 4" glass windows on new 13.2 KV Air switches that could be installed on metal cover for this enclosure. They are not cheap. Think maybe 8 years ago they cost over $400 each.
 
petersonra

petersonra

Senior Member
Location
Northern illinois
Occupation
engineer
I think people miss the difference between guarding to keep people from touching live parts and guarding that prevents arc blast from escaping an enclosure and harming someone outside of the enclosure.

The first is fairly straightforward. The second is not so simple.
 
S

Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
So, article 240.30 requires:
(A) Protection from Physical Damage. Overcurrent device
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shall be protected from physical damage by one of the follow‐
ing:
(1) Installation in enclosures, cabinets, cutout boxes, or
equipment assemblies
(2) Mounting on open-type switchboards, panelboards, or
control boards that are in rooms or enclosures free from
dampness and easily ignitible material and are accessible
only to qualified personnel
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Article 312.10(C) requires nonmetallic cabinets to be listed.

I would say a single breaker cabinet doesn't comply with 240.30 (A) (2), so it must be listed per 312.10(C)
 
J

jim dungar

Moderator
Staff member
Location
Wisconsin
Occupation
PE (Retired) - Power Systems
Strathead said:
So, article 240.30 requires:


Article 312.10(C) requires nonmetallic cabinets to be listed.

I would say a single breaker cabinet doesn't comply with 240.30 (A) (2), so it must be listed per 312.10(C)
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It is fine if it meets 240.30(A)(1).
240.30(A)(2) is about open/exposed equipment, which would not be applicable if an enclosure door is present as stated in the OP.
 
G

garbo

Senior Member
Jraef said:
A.K.A. “suicide rooms”…
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When I started out back in 1970 in an old plant they had what old timer called Frankenstein switches. They were two phase 100 to 400 amp open blade switches. ( copper blades were exposed ). Had a screened cage around them with warning signs. Luckily when I returned after serving in Viet Nam they got rid of them while changing over that part of the building to three phase. Scariest part was somebody wired one set of switches backwards probably back in the 1920's where one set of two phase ( 4 fuses ) fuse block were still energised when the Frankenstein switch was opened.
 
R

ramsy

Roger Ruhle dba NoFixNoPay
Location
LA basin, CA
Occupation
Service Electrician 2020 NEC
dbhennessey said:
..It is not a good idea for a possible arc flash. Is there anything in code..
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NEC 112(A) requires unused openings be protected equivalent to the wall of the enclosure.

NEC 110.27 & 31 lists ways to guard live parts, which may be what you need.
 
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